Sound waves that have a frequency greater than approximately 20 kHz are referred to in the art as “ultrasound.” In the medical field, ultrasound waves are useful for both diagnostic and therapeutic applications. Medical diagnostic ultrasound systems are useful for generating images of anatomical structures within a target area of a patient's body. The images are obtained by scanning a target area with waves of ultrasound energy. In therapeutic ultrasound applications, high intensity ultrasound energy is transmitted into a target area to induce changes in state of the target. High-intensity focused ultrasound (“HIFU”) pulses induce changes in tissue state through thermal effects (e.g., induced hyperthermia) and mechanical effects (e.g., induced cavitation).
The use of high intensity focused ultrasound to eliminate tissue or to alter the characteristics of tissue at a target location, volume, region or area within a larger mass, body or area of anatomical tissue presents many advantages, including minimization of trauma and pain for the patient, elimination of the need for a surgical incision, stitches and exposure of internal tissue, avoidance of damage to tissue other than that which is to be treated, altered or removed, lack of a harmful cumulative effect from the ultrasound energy on the surrounding non-target tissue, reduction in treatment costs, elimination of the need in many cases for general anesthesia, reduction of the risk of infection and other complications, avoidance of blood loss, and the ability for high intensity focused ultrasound procedures to be performed in non-hospital sites and/or on an out-patient basis.
In high-intensity focused ultrasound hyperthermia treatments, intensity of ultrasonic waves generated by a highly focused transducer increases from the source to the region of focus where it can reach a very high temperature. The absorption of the ultrasonic energy at the focus induces a sudden temperature rise of tissue, which causes ablation of the target volume of cells in the focal region. Thus, as an example, HIFU hyperthermia treatments can cause necrotization of an internal lesion without damage to the intermediate tissues. The focal region dimensions are referred to as the depth of field, and the distance from the transducer to the center point of the focal region is referred to as the depth of focus. Ultrasound is a promising non-invasive surgical technique because the ultrasonic waves provide effective penetration of intervening tissues, yet with sufficiently low attenuation to deliver energy to a small focal target volume. Currently there is no other known modality that offers noninvasive, deep, localized focusing of non-ionizing radiation for therapeutic purposes. Thus, ultrasonic treatment has a great advantage over electromagnetic and radioactive therapeutic treatment techniques.
The beam emitted by a single ultrasound focused transducer element is generally effective within a fixed region, called the “focal zone.” This focal zone frequently is smaller than the size of the target tissue. Treatment of extensive targets is consequently a problem. A solution to this shortcoming is to utilize a transducer comprising a plurality of individual transducer elements arranged closely together to form an array. These arrays are focused at the desired treatment site through a combination of geometric and electronic focusing. Geometric focusing is determined by the permanent geometry of the array, while electronic focusing involves the use of phase delays and wave interference to achieve constructive interference at the target tissue. Electronic focusing allows movement of the treatment location without the need for mechanical movement of the array.
A particular problem is the use of small ultrasound arrays in relation to certain types of medical treatments, such as HIFU treatment in association with laparoscopic, percutaneous, and interstitial procedures. Individual focused transducer elements are often smaller than the target tissue, requiring movement of the transducer to ablate the tissue. The use of transducer arrays is likewise problematic, since the space available for placement or insertion of the transducer is limited.
HIFU treatment using a small focal point, whether generated from a single ultrasound transducer element or a phased array, allows great treatment selectivity. For example, the focal spot can be used to treat particular regions of tissue while sparing critical tissue structures. However, focused transducers are slow when used to treat large volumes of tissue. It is desirable to provide an ultrasound transducer that is capable of providing a small, selective focal spot for treating a specific region of tissue and is also capable of treating large volumes of tissue.
A potential solution is the use of a physically small array having fewer transducer elements, wherein the elements can be electronically controlled to focus or steer the beam. A conventional ultrasound transducer array used for beam forming typically requires small transducer elements having an aperture pitch that is half the wavelength of the ultrasound signal or less. Ultrasound arrays for use in confined spaces preferably have larger transducer elements of two or more wavelengths in aperture pitch for effective ablation. Conventional methods of focusing and beam steering are not compatible with these transducers. When an electronic focus is attempted with such a transducer, large secondary off-axis foci, or “grating lobes” are created. These grating lobes are comparable in amplitude to the primary focus and draw energy from the main focal lobe, reducing the efficacy of the ultrasound beam.
Another potential solution is to use an unfocused ultrasound transducer. An unfocused ultrasound transducer will treat a larger portion of the target tissue. However, the ablation depth may be substantially reduced in relation to a focused ultrasound beam, necessitating more movement of the transducer during the ablation procedure.
Still, scientists and engineers continue to seek improved methods for therapeutic ultrasound medical treatment of tissue. There is a need for a physically small ultrasound array that is capable of treating a relatively large portion of target tissue with a greater ablation depth than is currently available in the art.